Rather than manually mount and demount removable media from a data recording device, it has been the practice in certain situations to provide an automatic media library. Such media library can be viewed as a specialized form of automatic warehousing, where the warehoused item is record media and where in the input/output are streams of data objects, as well as record media being inserted and removed from the library. The earliest form of record media libraries used magnetic recording tape and microfiche cards. Either the tape or the card libraries employed open-faced walls having a plurality of storage compartments, cells or slots for passively storing the magnetic tape/cards. Integrally associated with the library are one or more recording or reading devices. In the case of magnetic tape drives, the drives are either at the end of the wall or disposed and mounted inside the open-face array of storage cells. In both types of libraries, a travelling elevator automatically moved the record media between the passive storage cells and the data recording/reading devices. In the case of magnetic tape, a single unit of tape, either a cartridge or spool is transported by the travelling elevator during each motion. In a similar manner, a cartridge or package of microfiche cards could be transported between the passive cells and the microfiche reader/recorder as a single unit. In both types of early libraries, the motions of the travelling elevator were sequenced for minimizing the travel length and travel time for an increase in efficiency of the library system. In both types of libraries, the record media were assigned a home passive position. Upon command, the travelling elevator would fetch a record medium from its permanently assigned storage cell and transport it to a recorder. Upon completion of the data processing operation of recording or reading, the travelling elevator is then commanded to return the record medium back to its original storage cell. Provisions were made for inserting and removing various record media. The insertion or removal would take the removed media out of any catalog or directory that defined the location of the record media in the library while the insertion of a record medium would require adding the identification of its informational content to a directory.
J. Christian in U.S. Pat. No. 4,120,629 (J. Christian) shows an automatic warehousing system for magnetic media involved in the manufacturing process. The magnetic media units were assigned to differing storage cells in accordance with the state of manufacturing. However, in each state of manufacturing the magnetic media was always kept in the same or temporarily assigned storage cell. Accordingly, it is seen that it has been the practice to assign a record medium to a given storage cell for the duration of its activity with respect to an automatic warehouse type of library.
Cary, et al. in U.S. Pat. No. 4,608,679 (369-036) shows a travelling elevator capable of carrying two record media at a time. Either one of the two media may be stored in a cell or inserted into a drive or removed therefrom independently of the other record medium. Cary, et al. do not teach how to efficiently use this travelling elevator, hereinafter referred to as "a medium carrier".